ms_transform

Parse_transform that translates fun syntax into match specifications.

This module implements the parse_transform that makes calls to
ets and dbg:fun2ms/1 translate into literal
match specifications. It also implements the back end for the same
functions when called from the Erlang shell.

The translations from fun's to match_specs
is accessed through the two "pseudo
functions" ets:fun2ms/1 and dbg:fun2ms/1.

Actually this introduction is more or less an introduction to the
whole concept of match specifications. Since everyone trying to use
ets:select or dbg seems to end up reading
this page, it seems in good place to explain a little more than
just what this module does.

There are some caveats one should be aware of, please read through
the whole manual page if it's the first time you're using the
transformations.

Match specifications are used more or less as filters.
They resemble usual Erlang matching in a list comprehension or in
a fun used in conjunction with lists:foldl etc. The
syntax of pure match specifications is somewhat awkward though, as
they are made up purely by Erlang terms and there is no syntax in the
language to make the match specifications more readable.

As the match specifications execution and structure is quite like
that of a fun, it would for most programmers be more straight forward
to simply write it using the familiar fun syntax and having that
translated into a match specification automatically. Of course a real
fun is more powerful than the match specifications allow, but bearing
the match specifications in mind, and what they can do, it's still
more convenient to write it all as a fun. This module contains the
code that simply translates the fun syntax into match_spec terms.

Let's start with an ets example. Using ets:select and
a match specification, one can filter out rows of a table and construct
a list of tuples containing relevant parts of the data in these
rows. Of course one could use ets:foldl instead, but the
select call is far more efficient. Without the translation, one has to
struggle with writing match specifications terms to accommodate this,
or one has to resort to the less powerful
ets:match(_object) calls, or simply give up and use
the more inefficient method of ets:foldl. Using the
ets:fun2ms transformation, a ets:select call
is at least as easy to write as any of the alternatives.

As an example, consider a simple table of employees:

-record(emp, {empno, %Employee number as a string, the key
surname, %Surname of the employee
givenname, %Given name of employee
dept, %Department one of {dev,sales,prod,adm}
empyear}). %Year the employee was employed

Even though ets:match does not require a full match
specification, but a simpler type, it's still somewhat unreadable, and
one has little control over the returned result, it's always a list of
lists. OK, one might use ets:foldl or
ets:foldr instead:

Running that would result in ["011103","076324"]
, which at least gets rid of the extra lists. The fun is also quite
straightforward, so the only problem is that all the data from the
table has to be transferred from the table to the calling process for
filtering. That's inefficient compared to the ets:match
call where the filtering can be done "inside" the emulator and only
the result is transferred to the process. Remember that ets tables are
all about efficiency, if it wasn't for efficiency all of ets could be
implemented in Erlang, as a process receiving requests and sending
answers back. One uses ets because one wants performance, and
therefore one wouldn't want all of the table transferred to the
process for filtering. OK, let's look at a pure
ets:select call that does what the ets:foldr
does:

Even though the record syntax is used, it's still somewhat hard to
read and even harder to write. The first element of the tuple,
#emp{empno = '$1', dept = sales, _='_'} tells what to
match, elements not matching this will not be returned at all, as in
the ets:match example. The second element, the empty list
is a list of guard expressions, which we need none, and the third
element is the list of expressions constructing the return value (in
ets this almost always is a list containing one single term). In our
case '$1' is bound to the employee number in the head
(first element of tuple), and hence it is the employee number that is
returned. The result is ["011103","076324"], just as in
the ets:foldr example, but the result is retrieved much
more efficiently in terms of execution speed and memory consumption.

We have one efficient but hardly readable way of doing it and one
inefficient but fairly readable (at least to the skilled Erlang
programmer) way of doing it. With the use of ets:fun2ms,
one could have something that is as efficient as possible but still is
written as a filter using the fun syntax:

This may not be the shortest of the expressions, but it requires no
special knowledge of match specifications to read. The fun's head
should simply match what you want to filter out and the body returns
what you want returned. As long as the fun can be kept within the
limits of the match specifications, there is no need to transfer all
data of the table to the process for filtering as in the
ets:foldr example. In fact it's even easier to read then
the ets:foldr example, as the select call in itself
discards anything that doesn't match, while the fun of the
foldr call needs to handle both the elements matching and
the ones not matching.

It's worth noting in the above ets:fun2ms example that one
needs to include ms_transform.hrl in the source code, as this is
what triggers the parse transformation of the ets:fun2ms call
to a valid match specification. This also implies that the
transformation is done at compile time (except when called from the
shell of course) and therefore will take no resources at all in
runtime. So although you use the more intuitive fun syntax, it gets as
efficient in runtime as writing match specifications by hand.

Let's look at some more ets examples. Let's say one
wants to get all the employee numbers of any employee hired before the
year 2000. Using ets:match isn't an alternative here as
relational operators cannot be expressed there. Once again, an
ets:foldr could do it (slowly, but correct):

This gives the same result, the [{'<', '$2', 2000}] is in
the guard part and therefore discards anything that does not have a
empyear (bound to '$2' in the head) less than 2000, just as the guard
in the foldl example. Lets jump on to writing it using
ets:fun2ms

I'll show some more examples without the tiresome
comparing-to-alternatives stuff. Let's say we'd want the whole object
matching instead of only one element. We could of course assign a
variable to every part of the record and build it up once again in the
body of the fun, but it's easier to do like this:

Just as in ordinary Erlang matching, you can bind a variable to the
whole matched object using a "match in then match", i.e. a
=. Unfortunately this is not general in fun's translated
to match specifications, only on the "top level", i.e. matching the
whole object arriving to be matched into a separate variable,
is it allowed. For the one's used to writing match specifications by
hand, I'll have to mention that the variable A will simply be
translated into '$_'. It's not general, but it has very common usage,
why it is handled as a special, but useful, case. If this bothers you,
the pseudo function object also returns the whole matched
object, see the part about caveats and limitations below.

Let's do something in the fun's body too: Let's say
that someone realizes that there are a few people having an employee
number beginning with a zero (0), which shouldn't be
allowed. All those should have their numbers changed to begin with a
one (1) instead and one wants the
list [{<Old empno>,<New empno>}] created:

As a matter of fact, this query hits the feature of partially bound
keys in the table type ordered_set, so that not the whole
table need be searched, only the part of the table containing keys
beginning with 0 is in fact looked into.

The fun of course can have several clauses, so that if one could do
the following: For each employee, if he or she is hired prior to 1997,
return the tuple {inventory, <employee number>}, for each hired 1997
or later, but before 2001, return {rookie, <employee number>}, for all others return {newbie, <employee number>}. All except for the ones named Smith as
they would be affronted by anything other than the tag
guru and that is also what's returned for their numbers;
{guru, <employee number>}:

So, what more can you do? Well, the simple answer would be; look
in the documentation of match specifications in ERTS users
guide. However let's briefly go through the most useful "built in
functions" that you can use when the fun is to be
translated into a match specification by ets:fun2ms (it's
worth mentioning, although it might be obvious to some, that calling
other functions than the one's allowed in match specifications cannot
be done. No "usual" Erlang code can be executed by the fun being
translated by fun2ms, the fun is after all limited
exactly to the power of the match specifications, which is
unfortunate, but the price one has to pay for the execution speed of
an ets:select compared to ets:foldl/foldr).

The head of the fun is obviously a head matching (or mismatching)
one parameter, one object of the table we select
from. The object is always a single variable (can be _) or
a tuple, as that's what's in ets, dets and
mnesia tables (the match specification returned by
ets:fun2ms can of course be used with
dets:select and mnesia:select as well as
with ets:select). The use of = in the head
is allowed (and encouraged) on the top level.

The guard section can contain any guard expression of Erlang.
Even the "old" type test are allowed on the toplevel of the guard
(integer(X) instead of is_integer(X)). As the new type tests (the
is_ tests) are in practice just guard bif's they can also
be called from within the body of the fun, but so they can in ordinary
Erlang code. Also arithmetics is allowed, as well as ordinary guard
bif's. Here's a list of bif's and expressions:

Contrary to the fact with "handwritten" match specifications, the
is_record guard works as in ordinary Erlang code.

Semicolons (;) in guards are allowed, the result will be (as
expected) one "match_spec-clause" for each semicolon-separated
part of the guard. The semantics being identical to the Erlang
semantics.

The body of the fun is used to construct the
resulting value. When selecting from tables one usually just construct
a suiting term here, using ordinary Erlang term construction, like
tuple parentheses, list brackets and variables matched out in the
head, possibly in conjunction with the occasional constant. Whatever
expressions are allowed in guards are also allowed here, but there are
no special functions except object and
bindings (see further down), which returns the whole
matched object and all known variable bindings respectively.

The dbg variants of match specifications have an
imperative approach to the match specification body, the ets dialect
hasn't. The fun body for ets:fun2ms returns the result
without side effects, and as matching (=) in the body of
the match specifications is not allowed (for performance reasons) the
only thing left, more or less, is term construction...

Let's move on to the dbg dialect, the slightly
different match specifications translated by dbg:fun2ms.

The same reasons for using the parse transformation applies to
dbg, maybe even more so as filtering using Erlang code is
simply not a good idea when tracing (except afterwards, if you trace
to file). The concept is similar to that of ets:fun2ms
except that you usually use it directly from the shell (which can also
be done with ets:fun2ms).

As can be seen, the fun's used with
dbg:fun2ms takes a single list as parameter instead of a
single tuple. The list matches a list of the parameters to the traced
function. A single variable may also be used of course. The body
of the fun expresses in a more imperative way actions to be taken if
the fun head (and the guards) matches. I return true here, but it's
only because the body of a fun cannot be empty, the return value will
be discarded.

When we run the test of our module now, we get the following trace
output:

(<0.86.0>) call ets:new(toy_table,[ordered_set])

Let's play we haven't spotted the problem yet, and want to see what
ets:new returns. We do a slightly different trace
pattern:

The call to return_trace, makes a trace message appear
when the function returns. It applies only to the specific function call
triggering the match specification (and matching the head/guards of
the match specification). This is the by far the most common call in the
body of a dbg match specification.

As the test now fails with {badmatch,24}, it's obvious
that the badmatch is because the atom toy_table does not
match the number returned for an unnamed table. So we spotted the
problem, the table should be named and the arguments supplied by our
test program does not include named_table. We rewrite the
start function to:

start(Args) ->
toy_table = ets:new(toy_table,[named_table |Args]).

And with the same tracing turned on, we get the following trace
output:

Very well. Let's say the module now passes all testing and goes into
the system. After a while someone realizes that the table
toy_table grows while the system is running and that for some
reason there are a lot of elements with atom's as keys. You had
expected only integer keys and so does the rest of the system. Well,
obviously not all of the system. You turn on call tracing and try to
see calls to your module with an atom as the key:

We use dbg:tpl here to make sure to catch local calls
(let's say the module has grown since the smaller version and we're
not sure this inserting of atoms is not done locally...). When in
doubt always use local call tracing.

Let's say nothing happens when we trace in this way. Our function
is never called with these parameters. We make the conclusion that
someone else (some other module) is doing it and we realize that we
must trace on ets:insert and want to see the calling function. The
calling function may be retrieved using the match specification
function caller and to get it into the trace message, one
has to use the match spec function message. The filter
call looks like this (looking for calls to ets:insert):

You have found out that the function evil_fun of the
module evil_mod, with arity 2, is the one
causing all this trouble.

This was just a toy example, but it illustrated the most used
calls in match specifications for dbg The other, more
esotheric calls are listed and explained in the Users guide of the ERTS application, they really are beyond the scope of this
document.

To end this chatty introduction with something more precise, here
follows some parts about caveats and restrictions concerning the fun's
used in conjunction with ets:fun2ms and
dbg:fun2ms:

Warning!

To use the pseudo functions triggering the translation, one
has to include the header file ms_transform.hrl
in the source code. Failure to do so will possibly result in
runtime errors rather than compile time, as the expression may
be valid as a plain Erlang program without translation.

Warning!

The fun has to be literally constructed inside the
parameter list to the pseudo functions. The fun cannot
be bound to a variable first and then passed to
ets:fun2ms or dbg:fun2ms, i.e this
will work: ets:fun2ms(fun(A) -> A end) but not this:
F = fun(A) -> A end, ets:fun2ms(F). The later will result
in a compile time error if the header is included, otherwise a
runtime error. Even if the later construction would ever
appear to work, it really doesn't, so don't ever use it.

Several restrictions apply to the fun that is being translated
into a match_spec. To put it simple you cannot use anything in
the fun that you cannot use in a match_spec. This means that,
among others, the following restrictions apply to the fun itself:

Functions written in Erlang cannot be called, neither
local functions, global functions or real fun'sEverything that is written as a function call will be
translated into a match_spec call to a builtin function, so that
the call is_list(X) will be translated to {'is_list', '$1'} ('$1' is just an example, the numbering may
vary). If one tries to call a function that is not a match_spec
builtin, it will cause an error.Variables occurring in the head of the fun will be
replaced by match_spec variables in the order of occurrence, so
that the fragment fun({A,B,C}) will be replaced by
{'$1', '$2', '$3'} etc. Every occurrence of such a
variable later in the match_spec will be replaced by a
match_spec variable in the same way, so that the fun
fun({A,B}) when is_atom(A) -> B end will be translated into
[{{'$1','$2'},[{is_atom,'$1'}],['$2']}].

Variables that are not appearing in the head are imported
from the environment and made into
match_spec const expressions. Example from the shell:

All variables are bound in the head of a match_spec, so the
translator can not allow multiple bindings. The special case
when matching is done on the top level makes the variable bind
to '$_' in the resulting match_spec, it is to allow a more
natural access to the whole matched object. The pseudo
function object() could be used instead, see below.
The following expressions are translated equally:

The special match_spec variables '$_' and '$*'
can be accessed through the pseudo functions object()
(for '$_') and bindings() (for '$*').
as an example, one could translate the following
ets:match_object/2 call to a ets:select call:

ets:match_object(Table, {'$1',test,'$2'}).

...is the same as...

ets:select(Table, ets:fun2ms(fun({A,test,B}) -> object() end)).

(This was just an example, in this simple case the former
expression is probably preferable in terms of readability).
The ets:select/2 call will conceptually look like this
in the resulting code:

ets:select(Table, [{{'$1',test,'$2'},[],['$_']}]).

Matching on the top level of the fun head might feel like a
more natural way to access '$_', see above.

Term constructions/literals are translated as much as is
needed to get them into valid match_specs, so that tuples are
made into match_spec tuple constructions (a one element tuple
containing the tuple) and constant expressions are used when
importing variables from the environment. Records are also
translated into plain tuple constructions, calls to element
etc. The guard test is_record/2 is translated into
match_spec code using the three parameter version that's built
into match_specs, so that is_record(A,t) is translated
into {is_record,'$1',t,5} given that the record size of
record type t is 5.Language constructions like case, if,
catch etc that are not present in match_specs are not
allowed.If the header file ms_transform.hrl is not included,
the fun won't be translated, which may result in a
runtime error (depending on if the fun is valid in a
pure Erlang context). Be absolutely sure that the header is
included when using ets and dbg:fun2ms/1 in
compiled code.If the pseudo function triggering the translation is
ets:fun2ms/1, the fun's head must contain a single
variable or a single tuple. If the pseudo function is
dbg:fun2ms/1 the fun's head must contain a single
variable or a single list.

The translation from fun's to match_specs is done at compile
time, so runtime performance is not affected by using these pseudo
functions. The compile time might be somewhat longer though.

For more information about match_specs, please read about them
in ERTS users guide.

parse_transform(Forms, Options) -> Forms

Implements the actual transformation at compile time. This
function is called by the compiler to do the source code
transformation if and when the ms_transform.hrl header
file is included in your source code. See the ets and
dbg:fun2ms/1 function manual pages for
documentation on how to use this parse_transform, see the
match_spec chapter in ERTS users guide for a
description of match specifications.

transform_from_shell(Dialect, Clauses, BoundEnvironment) -> term()

BoundEnvironment = List of variable bindings in the shell environment.

Implements the actual transformation when the fun2ms
functions are called from the shell. In this case the abstract
form is for one single fun (parsed by the Erlang shell), and
all imported variables should be in the key-value list passed
as BoundEnvironment. The result is a term, normalized,
i.e. not in abstract format.